This invention relates generally to the cleaning of gas streams, and, more particularly, to the removal of solid particulate matter from the combustion gas stream of coal-fired power plants and other operations that produce particle-laden gas streams.
In a coal-fired power plant, coal is burned to heat air, which in turn boils water to form steam. The steam drives a turbine and thence an electric generator, producing electricity. Besides heat, the burning of the coal produces gaseous pollutants such as sulfur and nitrogen oxides, and a solid particulate known as fly ash. Environmental protection laws mandate that the amounts of gaseous pollutants and solid particulate emitted from the power plant be maintained at acceptably low levels, and the present invention deals generally with the technology for controlling particulate emissions.
One widely used approach for removing the particulate fly ash from combustion gas streams is electrostatic precipitation. The combustion gas stream with entrained particulate is passed between highly charged electrodes that ionize the particles so that they are deposited upon the collection electrodes. The particulate may optionally be charged prior to entry into the precipitator to increase the efficiency of removal. The cleaned combustion gases are released to the atmosphere, and the precipitated particulate is removed from the plates.
To control the sulfur levels in power plant emissions, coals containing low sulfur levels are sometimes burned. However, the particulate fly ash resulting from the burning of low-sulfur coal may be difficult to remove by electrostatic precipitation, because the electrical resistance of the particulate is too high for effective treatment. When high-sulfur coal is burned, sulfur trioxide naturally present in the particulate reacts with residual water to produce sulfuric acid that is deposited upon the surface of the particulate. The sulfuric acid produces ions which conduct electrical charge and reduce the surface electrical resistance of the particulate, permitting the use of the electrostatic precipitation treatment.
Since low-sulfur coal does not inherently produce sufficient sulfur trioxide to achieve the necessary electrical conductivity of the fly ash, it is sometimes necessary to add a controlled amount of sulfur trioxide to the combustion gas stream to condition the particulate so that it may be removed by electrostatic precipitation. The added sulfur trioxide reacts with water vapor in the gas stream to produce sulfuric acid, which is deposited upon the surface of the particulate. The amount of sulfur trioxide to be added is carefully controlled so that the cleaned combustion gas finally released to the atmosphere has a desirable balance of low sulfur content and low particulate content.
Several types of apparatus have been developed for controllably adding sulfur to condition the particulate of a flowing gas stream. One example of such an apparatus is that disclosed in U.S. Pat. No. 3,993,429. In the apparatus of that patent, sulfur is burned to a combustion product which is passed over a catalyst, and the resulting sulfur trioxide is injected into the flowing combustion gas stream in the correct proportion. This apparatus has been highly successful commercially, and is used in power plants throughout the United States and the world.
However, there are problems even with the sulfur trioxide-injection conditioning technology as presently practiced. First, the sulfur trioxide injection nozzles must be placed sufficiently far from the electrostatic precipitator that there is a retention time on the order of about one second between the point of injection of the sulfur trioxide into the flowing gas stream and the point that the stream enters the precipitator. This retention time permits the sulfur trioxide and water vapor to react and the resulting sulfuric acid to deposit upon the particulate. Since the combustion gas flow velocity is about 60 feet per second in a typical power plant, the sulfur injection nozzles must be about 60 feet from the precipitator, increasing the size of the plant and its capital cost.
Additionally, there are some types of low-sulfur coal that cannot be effectively conditioned by the known technology. An example is Australian Sydney Basin coal, which for a variety of reasons resists the deposition of sulfur trioxide and sulfuric acid onto its surface. The burning of such coals results in very low sulfur emission, but high levels of particulate.
There therefor exists a need for an improved combustion gas conditioning system that permits reduction in plant size, and increases the range of the types of coal that may be burned in power plants that have strict particulate emission standards. The present invention fulfills this need, and further provides related advantages.